Internet Engineering Task Force (IETF)                           J. Haas
Request for Comments: 7300                              Juniper Networks
BCP: 6                                                       J. Mitchell
Updates: 1930                                      Microsoft Corporation
Category: Best Current Practice                                July 2014
ISSN: 2070-1721


          Reservation of Last Autonomous System (AS) Numbers

Abstract

  This document reserves two Autonomous System Numbers (ASNs) at the
  end of the 16-bit and 32-bit ranges, described in this document as
  "Last ASNs", and provides guidance to implementers and operators on
  their use.  This document updates Section 10 of RFC 1930.

Status of This Memo

  This memo documents an Internet Best Current Practice.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It has been approved for publication by the Internet
  Engineering Steering Group (IESG).  Further information on BCPs is
  available in Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc7300.

Copyright Notice

  Copyright (c) 2014 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.







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1.  Introduction

  Over a decade ago, IANA reserved the last Autonomous System Number
  (ASN) of the 16-bit ASN range, 65535, with the intention that it not
  be used by network operators running BGP [RFC4271].  Since the
  introduction of "BGP Support for Four-Octet Autonomous System (AS)
  Number Space" [RFC6793], IANA has also reserved the last ASN of the
  32-bit autonomous system number range, 4294967295.  This reservation
  has been documented in the IANA "Autonomous System (AS) Numbers"
  registry [IANA.AS].  Although these "Last ASNs" border on Private Use
  ASN [RFC6996] ranges, they are not defined or reserved as Private Use
  ASNs by [IANA.AS].  This document describes the reasoning for
  reserving Last ASNs and provides guidance both to operators and to
  implementers on their use.

2.  Requirements Language

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Reasons for Reservation of the Last ASNs

  A subset of the BGP communities of ASN 65535, the last ASN of the
  16-bit range, are reserved for use by Well-known Communities as
  described in [RFC1997] and [IANA.WK].  Although this is not currently
  true of ASN 4294967295, if there is a future need for another Special
  Use ASN that is not designed to be globally routable, or for the
  associated BGP communities of such an ASN, ASN 4294967295 could be a
  valid candidate for such purpose.  This document does not prescribe
  any such Special Use to this ASN at the time of publication.

4.  Operational Considerations

  Operators SHOULD NOT use these Last ASNs for any other purpose or as
  Private Use ASNs.  Operational use of these Last ASNs could have
  undesirable results.  For example; use of AS 65535 as if it were a
  Private Use ASN, may result in inadvertent use of BGP Well-known
  Community values [IANA.WK], causing undesirable routing behavior.

  Last ASNs MUST NOT be advertised to the global Internet within
  AS_PATH or AS4_PATH attributes.  Operators SHOULD filter Last ASNs
  within the AS_PATH and AS4_PATH attributes.








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RFC 7300                   Last AS Reservation                 July 2014


5.  Implementation Considerations

  While Last ASNs are reserved, they remain valid ASNs from a BGP
  perspective.  Therefore, implementations of BGP [RFC4271] SHOULD NOT
  treat the use of Last ASNs as any type of protocol error.  However,
  if a Last ASN is configured as the local AS, implementations MAY
  generate a warning message indicating improper use of a reserved ASN.

  Implementations that provide tools that filter Private Use ASNs
  within the AS_PATH and AS4_PATH attributes MAY also include Last
  ASNs.

6.  IANA Considerations

  IANA has reserved last Autonomous System number 65535 from the
  "16-bit Autonomous System Numbers" registry for the reasons described
  in this document.

  IANA has also reserved last Autonomous System number 4294967295 from
  the "32-bit Autonomous System Numbers" registry for the reasons
  described in this document.

  These reservations have been documented in the IANA "Autonomous
  System (AS) Numbers" registry [IANA.AS] and the IANA "Special-Purpose
  Autonomous System (AS) Numbers" registry [IANA.SpecialAS].

7.  Security Considerations

  This document does not introduce any additional security concerns in
  regards to usage of Last ASNs.  Although the BGP is designed to allow
  usage of Last ASNs, security issues related to BGP implementation
  errors could be triggered by Last ASN usage.

8.  References

8.1.  Normative References

  [IANA.AS]  IANA, "Autonomous System (AS) Numbers",
             <http://www.iana.org/assignments/as-numbers/>.

  [IANA.SpecialAS]
             IANA, "Special-Purpose Autonomous System (AS) Numbers",
             <http://www.iana.org/assignments/
             iana-as-numbers-special-registry/>.

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.




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RFC 7300                   Last AS Reservation                 July 2014


  [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
             Protocol 4 (BGP-4)", RFC 4271, January 2006.

  [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
             Autonomous System (AS) Number Space", RFC 6793, December
             2012.

8.2.  Informative References

  [IANA.WK]  IANA, "Border Gateway Protocol (BGP) Well-known
             Communities", <http://www.iana.org/assignments/
             bgp-well-known-communities/>.

  [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
             Communities Attribute", RFC 1997, August 1996.

  [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
             Private Use", BCP 6, RFC 6996, July 2013.

































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RFC 7300                   Last AS Reservation                 July 2014


Appendix A.  Acknowledgments

  The authors would like to thank Michelle Cotton and Elwyn Davies for
  encouraging the proper documentation of the reservation of these
  ASNs, and David Farmer for his contributions to the document.

Authors' Addresses

  Jeffrey Haas
  Juniper Networks

  EMail: [email protected]


  Jon Mitchell
  Microsoft Corporation
  One Microsoft Way
  Redmond, WA  98052
  USA

  EMail: [email protected]






























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